The focus of our work is a study of the intracellular transport of iron in the rabbit reticulocyte. Previous studies in this laboratory have investigated the mobilization of iron from specifically labeled Fe59-ghosts prepared from rabbit reticulocytes previously incubated with Fe59-transferrin-CO. 2 minus 3. The Fe59-ghosts are incubated with unlabeled lysate, various reagents, or a combination of lysate and reagents. The kinetics of iron mobilization is followed by removing aliquots at various times and rapidly cooling to 4 degrees. Centrifugation of the quenched mixture allows a separation and analysis of the Fe59-ghosts fraction and supernatant. Chromatography on Sephadex G-100 allows an analysis of the fate of the mobilized iron. We have found iron incorporation into ferritin, hemoglobin, an 18,000 molecular weight fraction, and a 6000 molecular weight component which we have isolated and partially characterized. We hope to develop an isolation procedure that will allow us to obtain the iron binding agent in milligram quantities. Gel chromatography and ultrafiltration techniques are most promising in this regard. We hope to characterize the physical and chemical properties of siderochelin. We are specifically interested in its exact molecular weight, the number of iron binding sites per mole, the oxidation state of the bound iron, the amino acid composition, and its possible relationship to metallothionine. We also wish to study the reversible binding of iron by this protein molecule. Third, we hope to elucidate its biological role.